Si el byte nº 25 es un 40h el ejecutable es para Windows.
Para mirarlo podríamos usar una función del tipo :

Function WinExe (ByVal Exe As String) As Integer
Dim fh As Integer
Dim t As String * 1

   fh = FreeFile
   Open Exe For Binary As #fh
      Get fh, 25, t
   Close #fh
   WinExe = (Asc(t) = &H40&)
End Function